The goal of this application is to understand the connectivity and function of a unique dual-transmitter amacrine cell circuit, in an effort to gain a comprehensive understanding of the functional organization of local neuronal circuits in the inner mammalian retina. Our recent studies discovered a number of intriguing synaptic and dendritic properties of the vGluT3 amacrine cell (GAC) network (Lee et al., Neuron 84:1049-64, 2014; Lee et al. Neuron 90: 27-34, 2016; Chen et al., PNAS 114:11518-11523, 2017), which offer an exciting opportunity to use this new model circuit to address many fundamental, long-standing questions regarding amacrine cell circuitry and function. Here, we propose to take a systematic approach to uncovering the connectivity and function of the entire local circuit of GACs, from the inputs from upstream bipolar cell types to the outputs to distinct downstream ganglion cell and amacrine cell types. The proposed study will focus on identifying new synaptic circuits between GACs and their synaptic partners and uncovering novel functional roles of GACs in shaping the receptive field properties of their downstream targets. Using the GAC circuit as a central link, we also aim to understand how the GAC circuit connectivity may help delineate the organization of other local circuits in the retina. There are three specific aims in this proposal: (1) to understand the synaptic connectivity and synaptic interactions between cone bipolar cell (CBC) types and GACs, (2) to understand function of a new glycinergic output circuit from GACs to ganglion cells, (3) to understand network interactions between GACs and other amacrine cell types. We will take an innovative approach to this goal by combining state-of-art electrophysiological (dual patch clamp in the inner nuclear layer of the wholemount mouse retina), optical (simultaneous two-photon imaging of local dendritic responses and patch clamp), genetic (in vivo viral transfection of genes in selected single neurons), and optogenetic techniques in a robust whole-mount preparation of vGluT3-Cre mouse retina. These techniques have been well tested in our recent studies and have helped us generate a substantial amount of preliminary data that are critical for the proposed study. The combination of these powerful tools with a unique, well-defined preparation offers an opportunity rarely attainable in other parts of the CNS. Results from this proposed research will shed important light on retinal circuit organization and function in health and disease.